Usually –tion words, such as motion, education, and lotion, end with a -shn sound. But equation ends with a sound rhyming with vision.
Are there many more? What might some of them be? And if the pronunciation of equation is a rarity for words ending in -tion, why did that pronunciation for it arise? I'm asking this since I am not a native English speaker and discovered this exception accidentally.
Best Answer
I've read through all of the words beginning with a through c in WS2's very useful list of -tion words, and so far I've found that the vast majority of the words in the -tion family carry a sh sound at the beginning of the final syllable.
The main exceptions to that pattern are some words ending in -stion (bastion, combustion, congestion, counterquestion, countersuggestion, etc.) or in -ntion (attention, contention, convention, circumvention, etc.), which instead carry (in typical U.S. English) a ch sound at the beginning of the final syllable.
There is also a red herring in the form of cation, which is of course not a -tion ending at all, but a cat[a]- prefix attached to the root word ion.
Most significantly to the point of the OP's original question, none of the a through c words in WS2's list carries the sound zh at the beginning of the final syllable.
[One hour later...] I finished reading through all of the -tion entries at MoreWords.com, and the only one that—in my generic U.S. English pronunciation—has a zh sound at the beginning of its final syllable is equation. I should have taken Peter Shor's word (in a comment above) for it.
I did come across the variant spelling kation to go with cation, and I encountered one interesting exception to the -ntion exception that I noted earlier: To my ear, at least, dissention carries a sh sound (and not a ch sound) at the beginning of its final syllable; but dissention is a bit weird anyway because it is a variant of the more common spelling dissension.
Anyway, I am fairly confident that the answer to the question "How many -tion words are there whose last syllable sounds like the last syllable in vision?" is one.